Optical recording, a technique utilizing a focused laser beam to make micron size marks in an appropriate medium for high density information recording, has been extensively studied in recent years. There are basically two types of optical recording; write-once and the erasable. In write-once recording, the media can only be recorded once, but the recorded information can be read many times. In erasable recording, the recorded information can be erased and new information can be recorded over the same are of the media.
There are several commercially available write-once optical recording products, but the introduction of erasable products has been plagued with delays. One of the major difficulties has been the availability of good media.
The technique most widely studied for erasable recording has been based on magneto-optic materials. This technique relies on the thermal-magnetic recording process. A focused laser beam is used to heat a spot on a magneto-optical material so that its coercivity is reduced and the magnetization within the spot can be switched by an applied field. The readout is accomplished by sensing the Kerr rotation of a reading laser beam induced by the magnetization in the media. Good recording performance has been reported by many working in the field. However, all reports are based on rare-earth/transition metal alloys, notably TbFeCo, and these alloys have some fundamental problems.
First of all, these materials are corrosion prone. Various approaches to solve the problem have been investigated, including the addition of a fourth element and the use of protecting layers on both sides of the alloy layer. The success of the first method is reducing the corrosion rate has been very limited; and it is also difficult to obtain defect free protecting layers to provide protection over the entire alloy layer. The compatibility of protecting layers with the alloy layers, in terms of adhesion, differential thermal expansion, etc., has been a problem also. Another problem with the alloy is that the properties critical to the optical recording process are extremely sensitive to the composition of the alloys. A few percent deviation from the optimum composition can degrade the performance significantly.
An alternative technique for erasable recording uses amorphous-crystalline phase-change materials. In this technique, a focused laser beam is used to switch the material between the amorphous state and the crystalline state. As is commonly done, a high power laser is used to heat a spot on the material to above its melting point to randomize the atomic arrangement in the material. When the laser beam is switched off, the material is left in the metastable amorphous state because of the high cooling rate. A low power laser, in many cases of longer duration, is then used to heat the material to below the melting point. The increased mobility of the atoms at the elevated temperature then allows the material to go to the more stable crystalline state. Thus by varying the power and duration of the laser beam, the material can be switched between the amorphous state and the crystalline state, and erasable recording is thus accomplished.
The major problem in the development of this technique has been the lack of appropriate materials. In particular, it has been difficult to find materials which have crystallization rate high enough under laser heating to alloy high rate recording (erasure time &lt;1 .mu.s), and yet slow enough at room temperature to ensure data integrity.
With slower erasing materials, the erase beam spot is normally made elliptical. This means that two lasers are needed in the recorder head. With faster erasing materials, only one laser, providing a circular spot, is needed in the recording head. The simplicity and cost advantage of a one-laser head over a two-laser head is apparent. Also, lower power laser pulse means lower laser cost and shorter laser pluse means higher data rate. In addition, low power laser pulse is less likely to damage the substrate. It is evident from the above discussions that super-sensitive media will offer many advantages.
EP-A1-0-212-336 describes a method of erasable recording using single-phase phase-change alloys. Whereas the crystallization rate of the preferred material, (GeTe).sub.85 Sn.sub.15, appeared to be high (erasure time &lt;55 ns), the laser power required for write and erase was also high (18 mW and 10 mW, respectively). While there was no mention of the corrosion resistance of the material, it contains a high concentration of corrosion prone tellurium.
Shogo Yagi, et al (Crystallization of Amorphous Marks in SbTe Erasable Optical Storage Media, Japanese Journal of Applied Physics, Vol 26, Supplement 26-4, 51(1987)), studied Sb.sub.x Te.sub.1-x erasable optical storage alloy. The composition range they reported were from x=0.19 to 0.55. They found that Sb.sub.2 Te.sub.3 thin film was the best composition due to its long amorphous life time at room temperature and short erasure time (.about.1 .mu.s).
Yagi's data indicate that when Sb content is higher than 55% the amorphizing threshold increases abruptly (FIG. 4 in Yagi's paper), and when Sb content is more than 44% the erasure time shows a trend of rapid increase (FIG. 8 is Yagi's paper).
The problem with the compositions of Yagi et al is that the time required for erasure are longer than desired and that the environmental stability of the composition is less than desired. The present invention is directed to a solution to these problems.